Mechanically robust polyacrylamide/gelatin ionic hydrogels reinforced by sodium alginate for wearable device applications

Abstract

Herein, an ionic hydrogel using sodium alginate (SA) toughened polyacrylamide (PAM)/gelatin semi-interpenetrating network with both high strength and high ductility for stress sensing is constructed. In the designed PGS-Ca²⁺/LiCl (short for PAM/Gelatin/SA-Ca²⁺/LiCl) hydrogel network, PAM acts as a flexible hydrophilic skeleton, and gelatin acts as a flexible secondary network. The addition of SA inhibits the phase separation of gelatin and improves the transparency of hydrogel. Meanwhile, the macromolecule SA complexes with metal ions of Ca²⁺, leading to the formation of a distinct complex structure which remarkably enhances the mechanical robustness of the hydrogel. Moreover, the incorporation of inorganic salt LiCl confers high electrical conductivity, concomitantly reducing the freezing point, mitigating water loss, and enhancing the environmental stability of the hydrogel, thereby endowing the hydrogel with improved adaptability to diverse operating conditions. PGS-Ca²⁺/LiCl has excellent mechanical properties and ultra-high ductility (with a tensile strength up to 110 kPa at break, a strain up to 1500% at break), as well as high stress sensing properties (with an excellent GF of 1.07, a pressure sensitivity of 0.0107). In addition, a handwriting sensor, a Morse code sensor, and an 8 × 8 sensor have been designed to recognize different signals and show the movement of objects and different pressures, which shows that PGS-Ca²⁺/LiCl ionic hydrogels have great potential in electronic skin, wearable and flexible devices.